The internal perfusion of squid axons with colchicine reversibly and selectively reduces the transient sodium current and the birefringence response to a brief depolarizing voltage pulse.
Recent studies(l,2) have shown that the nematocyst toxin of Physalia physalis (Portuguese Man-of-war) specifically affects the conduction system in both the neurogenic heart of the land cralb (Cardisoma guanhumi) and the myogenic heart of the rat. In earlier experiments the toxin was dissolved and injected immediately because of its extreme lability. Recently a more stable preparation of the toxin has been made, using a phosphate buffer (pH 6.3) that has prolonged activity sufficiently to allow us to observe the response to gradced doses.The present experiments were designed to study the effects of graded doses of P. physalis toxin upon the canine cardiovascular system, with particular emphasis on the EKG, systemic blood pressure, cardiac output, peripheral vascular resistance, and the levels of serum potassium and sodium. Effects of potassium infusion on toxin-induced arrhythmias were also evaluated.Methods. Toxin was prepared as previously described ( 3 ) , with the exception that the nematocysts were homogenized in 5 ml od isotonic Sorensen's phosphate buffer (containg 18.94 g Na2HP04; 18.16 g KH2P04 ; per liter). Following centrifugation (37,000 X g at 4OC for 30 min) the supernatant solution was lyophilized and stored in the cold in evacuated containers. Just prior to infusion, the lyophilized toxin was dissolved in 5% (w/v) dextrose buffered to pH 6.3 to a final concentration of 40 pg/ml. Eight mongrel dogs, 9.6-11.0 kg, were used in this experiment. The dogs were anesthetized with sodium pentobarbital, 28.6 mg/kg, administered intravenously after a 24-hour fasting period. The left carotid artery was ex-
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